Natural gas contains various hydrocarbons, including methane, ethane and propane. Natural gas usually has a major proportion of methane and ethane, i.e. methane and ethane together typically comprise at least 50 mole percent of the gas. The gas also contains relatively lesser amounts of heavier hydrocarbons such as propane, butanes, pentanes and the like, as well as hydrogen, nitrogen, carbon dioxide and other gases. In addition to natural gas, other gas streams containing hydrocarbons may contain a mixture of lighter and heavier hydrocarbons. For example, gas streams formed in the refining process can contain mixtures of hydrocarbons to be separated. Separation and recovery of these hydrocarbons can provide valuable products that may be used directly or as feedstocks for other processes. These hydrocarbons are typically recovered as natural gas liquids (NGL).
The present invention is primarily directed to recovery of C3+ components in gas streams containing hydrocarbons, and in particular to recovery of propane from these gas streams. A typical natural gas feed to be processed in accordance with the processes described below typically may contain, in approximate mole percent, 92.12% methane, 3.96% ethane and other C2 components, 1.05% propane and other C3 components, 0.15% iso-butane, 0.21% normal butane, 0.11% pentanes or heavier, and the balance made up primarily of nitrogen and carbon dioxide. Refinery gas streams may contain less methane and higher amounts of heavier hydrocarbons.
Recovery of natural gas liquids from a gas feed stream has been performed using various processes, such as cooling and refrigeration of gas, oil absorption, refrigerated oil absorption or through the use of multiple distillation towers. More recently, cryogenic expansion processes utilizing Joule-Thompson valves or turbo expanders have become preferred processes for recovery of NGL from natural gas.
In a typical cryogenic expansion recovery process, a feed gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of refrigeration such as a propane compression-refrigeration system. As the gas is cooled, liquids may be condensed and collected in one or more separators as high pressure liquids containing the desired components.
The high-pressure liquids may be expanded to a lower pressure and fractionated. The expanded stream, comprising a mixture of liquid and vapor, is fractionated in a distillation column. In the distillation column volatile gases and lighter hydrocarbons are removed as overhead vapors and heavier hydrocarbon components exit as liquid product in the bottoms.
The feed gas is typically not totally condensed, and the vapor remaining from the partial condensation may be passed through a Joule-Thompson valve or a turbo expander to a lower pressure at which further liquids are condensed as a result of further cooling of the stream. The expanded stream is supplied as a feed stream to the distillation column.
A reflux stream is provided to the distillation column, typically a portion of partially condensed feed gas after cooling but prior to expansion. Various processes have used other sources for the reflux, such as a recycled stream of residue gas supplied under pressure.
While various improvements to the general cryogenic processes described above have been attempted, these improvements continue to use a turbo expander or Joule-Thompson valve to expand the feed stream to the distillation column. It would be desirable to have an improved process for enhanced recovery of NGLs from a natural gas feed stream.